The objective of this research are (1) to determine whether there are fundamental differences in the development of thyroid function and its control by the hypothalamic-pituitary axis in precocial vs altricial modes of development, and (2) to compare the role of the thyroid gland in the development of thermoregulatory control in animals with precocial vs altricial patterns of development. This proposal addresses only the first of these objectives. We will use Japanese quail (precocial) and Ringed doves (altricial) as model systems for our comparisons. These species are equal in adult size and incubation time but differ markedly in development at hatching and in metabolic responses after hatching. Avian embryos are readily accessible to experimentation and they lack continuing maternal influence. There is a precocial-altricial range which spans the pattern of human development so they may provide basic information useful in later medical studies. Thyroid functional development will be compared in quail and doves from mid-incubation to juvenile attainment of homeothermy. We will measure thyroidal iodine trapping, hormone production/storage and release, and circulating concentrations of thyroxine (T4) and triiodothyroxine (T3). The relative roles of the thyroid gland vs peripheral tissues in altering hormone concentrations and T3/T4 ratios will be measured to explain the hormonal pattern through development. We will investigate whether the onset of pituitary control of the thyroid is limited by thyroid sensitivity to thyroid stimulating hormone (TSH) or by the development of pituitary capacity to produce/release TSH. The pituitary role in alterations in thyroid hormone concentrations and ratios during development will be evaluated. Maturation of the pituitary feedback system senstitive to thyroid hormones will be investigated by studies of thyroid hormone receptors. Hypothalamic control of the pituitary by thyrotropin releasing factor will also be investigated.